# resolver(3)

## NAME

         resolver,  res_query,  res_search,   res_mkquery,   res_send,   res_init,
         dn_comp, dn_expand - resolver routines

## SYNOPSIS

         #include <sys/types.h>
         #include <net/gen/in.h>
         #include <net/gen/nameser.h>
         #include <net/gen/resolv.h>

         res_query(dname, class, type, answer, anslen)
         char *dname;
         int class, type;
         u_char *answer;
         int anslen;

         res_search(dname, class, type, answer, anslen)
         char *dname;
         int class, type;
         u_char *answer;
         int anslen;

         res_mkquery(op, dname, class, type, data, datalen, newrr, buf, buflen)
         int op;
         char *dname;
         int class, type;
         char *data;
         int datalen;
         struct rrec *newrr;
         char *buf;
         int buflen;

         res_send(msg, msglen, answer, anslen)
         char *msg;
         int msglen;
         char *answer;
         int anslen;

         res_init()

         dn_comp(exp_dn, comp_dn, length, dnptrs, lastdnptr)
         char *exp_dn, *comp_dn;
         int length;
         char **dnptrs, **lastdnptr;

         dn_expand(msg, eomorig, comp_dn, exp_dn, length)
         char *msg, *eomorig, *comp_dn, exp_dn;
         int length;

## DESCRIPTION

         These routines are used for making, sending and  interpreting  query  and
         reply messages with Internet domain name servers.

         Global configuration and state information that is used by  the  resolver
         routines  is  kept  in  the  structure  _res.   Most  of  the values have
         reasonable defaults and can be ignored.  Options stored  in  _res.options
         are  defined  in  resolv.h  and  are as follows.  Options are stored as a
         simple bit mask containing the bitwise ``or'' of the options enabled.

         RES_INIT
              True if the initial name server address and default domain name  are
              initialized (i.e., res_init has been called).

         RES_DEBUG
              Print debugging messages.

         RES_AAONLY
              Accept authoritative  answers  only.   With  this  option,  res_send
              should  continue  until it finds an authoritative answer or finds an
              error.  Currently this is not implemented.

         RES_USEVC
              Use TCP connections for queries instead of UDP datagrams.

         RES_STAYOPEN
              Used with RES_USEVC to keep the TCP connection open between queries.
              This is useful only in programs that regularly do many queries.  UDP
              should be the normal mode used.

         RES_IGNTC
              Unused currently (ignore truncation errors, i.e., don't  retry  with
              TCP).

         RES_RECURSE
              Set the recursion-desired bit in  queries.   This  is  the  default.
              (res_send  does not do iterative queries and expects the name server
              to handle recursion.)

         RES_DEFNAMES
              If set, res_search will append the default domain  name  to  single-
              component  names  (those that do not contain a dot).  This option is
              enabled by default.

         RES_DNSRCH
              If this option is set, res_search will search for host names in  the
              current domain and in parent domains; see hostname(7).  This is used
              by the standard host lookup routine gethostbyname(3).   This  option
              is enabled by default.

         The  res_init  routine  reads  the  configuration  file  (if   any;   see
         resolver(5)) to get the default domain name, search list and the Internet
         address of the local name server(s).  If no  server  is  configured,  the
         host  running  the resolver is tried.  The current domain name is defined
         by the hostname if not specified in the configuration  file;  it  can  be
         overridden  by  the  environment  variable  LOCALDOMAIN.   Initialization
         normally occurs on the first call to one of the following routines.

         The  res_query  function  provides  an  interface  to  the  server  query
         mechanism.  It constructs a query, sends it to the local server, awaits a
         response, and makes preliminary checks on the reply.  The query  requests
         information  of  the  specified  type  and class for the specified fully-
         qualified domain name dname . The reply message is  left  in  the  answer
         buffer with length anslen supplied by the caller.

         The  res_search  routine  makes  a  query  and  awaits  a  response  like
         res_query,  but  in  addition, it implements the default and search rules
         controlled by the RES_DEFNAMES and RES_DNSRCH options.   It  returns  the
         first successful reply.

         The remaining routines are lower-level routines used by  res_query.   The
         res_mkquery function constructs a standard query message and places it in
         buf.  It returns the size of the query, or -1 if the query is larger than
         buflen.   The query type op is usually QUERY, but can be any of the query
         types defined in <arpa/nameser.h>.  The domain  name  for  the  query  is
         given  by  dname.   Newrr  is currently unused but is intended for making
         update messages.

         The res_send routine sends a pre-formatted query and returns  an  answer.
         It will call res_init if RES_INIT is not set, send the query to the local
         name server, and handle timeouts and retries.  The length  of  the  reply
         message is returned, or -1 if there were errors.

         The dn_comp function compresses the domain name exp_dn and stores  it  in
         comp_dn.  The size of the compressed name is returned or -1 if there were
         errors.  The size of the array pointed to by comp_dn is given by  length.
         The compression uses an array of pointers dnptrs to previously-compressed
         names in the current  message.   The  first  pointer  points  to  to  the
         beginning  of  the message and the list ends with NULL.  The limit to the
         array is specified by lastdnptr.  A side effect of dn_comp is  to  update
         the  list of pointers for labels inserted into the message as the name is
         compressed.  If dnptr is NULL, names are not compressed.  If lastdnptr is
         NULL, the list of labels is not updated.

         The dn_expand entry expands the compressed domain name comp_dn to a  full
         domain name The compressed name is contained in a query or reply message;
         msg is a pointer to the beginning of the message.  The uncompressed  name
         is placed in the buffer indicated by exp_dn which is of size length.  The
         size of compressed name is returned or -1 if there was an error.

## FILES

         /etc/resolv.conf see resolver(5)

## SEE ALSO

         gethostbyname(3), named(8), resolver(5), hostname(7),
         RFC1032, RFC1033, RFC1034, RFC1035, RFC974,
         SMM:11 Name Server Operations Guide for BIND